This invention generally relates to an improved method of electrodepositing a water-soluble or water-dispersible coating resin onto a conductive surface, and, in particular, is directed to a method and system for controlling the temperature of the electrocoating bath within the temperature range of about 90.degree.-135.degree. F.
The electrodeposition of water-based coatings commonly termed electrocoating is a widely used process which has many advantages over other methods of coating, such as spraying, dipping, rolling and the like. The advantages of electrocoating are well known. The process deposits a film of uniform thickness on essentially any conductive surface, even those which have sharp points and edges. Electrocoated film when applied is relatively water free and thus will not run or drip when taken out of the bath. Because little or no organic solvents are used in the resin system, the process is essentially fumeless and requires no extensive fume collection and incineration equipment. This latter point is important in view of the increased concern over environmental pollution. An additional advantage is the fact that a second or top coat can be applied over the electrocoated film without curing the electrocoated film and then both coats can be cured in one baking operation. By limiting the necessity of two baking furnaces, the cost of a two-coat process can be considerably reduced.
Electrocoating process generally comprises immersing the article to be coated into the electrocoating bath, usually as the anode, and passing a current through the bath between the article and electrode. The process usually is self-arresting in that as the thickness of the coating increases, the resistance thereof also increases, thereby limiting the amount of coating which is deposited.
During electrocoating, a considerable quantity of heat is generated in the bath which must be removed or dissipated in some manner so as to maintain the bath temperature at the desired level. Most commercial electrocoating installations, which maintain the electrocoating bath temperature at around ambient temperature, have extensive refrigeration equipment to maintain the bath temperature at around ambient levels. A higher temperature bath, e.g., 90 to about 135, would be more desirable because the higher temperature bath would allow for the use of tap water as a coolant and moreover provide a more efficient electrocoating process due to the acceleration of electroendosmosis at the higher temperatures. Notwithstanding the advantages of a high temperature bath, most commercial facilities employ a room temperature bath to eliminate the problems associated with preheating the bath prior to the start of electrocoating. It was found that heating elements tended to be quickly coated with a cured or semicured layer of resin which rendered the heating element more or less ineffective and frequently useless. It would be conceivable to utilize the heat generated during electrocoating to bring the bath temperature up to the desired level, but all of the material electrocoated during this period would have to be scrapped due to its low quality. In electrocoating processes wherein there is a large bath volume-to-workpiece area ratio, preheating the bath in this manner to the higher level would be prohibitive due to the excessive amounts of scrap which would be generated. Against this background, the present invention was developed.